The present invention relates to micro-optical elements for high power diode lasers and in particular, though not exclusively, to a monolithic fast-axis collimator array.
High power diode lasers are used in applications such as pumping of solid state lasers and directly in materials processing. In order to achieve the required power levels in a compact package, diode bars of emitters are arranged in stacks providing a two-dimensional array of emitters. The diode bars are typically a 10 mm long semiconductor bar fabricated with 19 to 79 emitters having widths of 100 to 200 μm. Fill factors typically range from 10% to 80% as a result of different combinations of emitter width and emitter pitch. For CW stacks the bar pitch is in the range 1 to 2 mm, while for QCW stacks pitch can be smaller, in the range 300 μm to 2 mm. The semiconductor is solder-bonded to a heat sink which can include channels for water cooling. A common configuration of commercially produced units stacks ten bars, each bar emitting 50-100 W, to build up a total laser power of 500-1000 W. Power levels up to 500 W per bar are achievable on QCW systems.
While such an arrangement produces high power, the beam quality is unacceptable for many applications. For high-brightness applications and also for some medium-brightness applications, by which we mean those with divergence well below the ex-facet divergence, the beam must be, at least, collimated. Manufacturers typically attach an individual fast-axis collimator to each bar. The most common fast-axis collimators are plano-acylindrical lenses, which are used to provide low aberration collimation for the high numerical aperture fast-axis beam, and cylindrical rod lenses, which provide poorer quality collimation at lower cost. The fast-axis refers to the vertical axis (perpendicular to the semiconductor wafer) where the beam diverges quickly (NA˜0.5) from an emitter region in the μm range. This is in contrast to the slow-axis, parallel to the face of the bars, where the emitter region is more typically 100 μm (NA 0.05-0.1). The slow axis (x-axis) and the fast axis (y-axis) are perpendicular to each other and orthogonal to the propagation direction of the beam (z-axis).
For many applications, the resultant beam quality is still insufficient. The disadvantages in using a plano-acylindrical lens along each bar are apparent as: the collimation lens cannot be correctly positioned for all points along the bar as a result of the “smile” effect, where the semiconductor bar is bent in the fast-axis direction by differential expansion during solder bonding, resulting in beams with variable pointing direction; the “facet bending effect”, where the semiconductor bar is bent in the direction of propagation again by the mechanical of the solder bonding process, resulting in beams with varying residual focal power, and errors in attaching the fast-axis collimator to the heat sink with the required positional accuracy also degrade the angular spectrum of the emitted light. Further, surface form errors in the acylindrical lens introduce aberrations that manifest themselves as wavefront distortion in the near field (i.e. the collimated wavefront is not perfectly flat) and divergence increase, leading to radiance loss, in the far-field. Additionally, in many applications of the laser diode stacks, subsequent aperture filling, beam shaping and beam combining optics are required and, due to errors in ray angles from the fast-axis collimator, the design and effectiveness of subsequent beam conditioning optics is compromised.
When the fast-axis bar pitch is smaller than ˜1.2 mm, aligning and fitting individual collimation lenses becomes mechanically difficult, due to the small amount of space available for holding, adjusting, and fixing down each lens. When bar pitch is higher than ˜1.2 mm, the same problems arise when high fill-factor collimation is required, since this requires the collimation lenses to be placed close together. Interleaving has been proposed (U.S. Pat. No. 6,993,059, U.S. Pat. No. 7,006,549, U.S. Pat. No. 6,266,359, U.S. Pat. No. 7,680,170) to achieve a fast-axis collimated beam with high fill-factor. Interleaving combines two low fill-factor beams using an interleaver optical element. The main disadvantage with this solution is that the interleaver element is expensive to manufacture and takes up a significant amount of space. Additionally most interleaver arrangements require two separate laser diode bar stacks. An alternative solution is to compact a single low fill-factor beam with a step mirror or alternative optic with a similar function. Such a mirror is described in U.S. Pat. No. 6,240,116. This solution has similar disadvantages to those of the interleaver element.
In an attempt to overcome some of these problems, Doric Lenses Inc., Quebec, Canada, have developed a fast-axis collimator lens array assembly using discrete fast-axis collimators located in a holder at the end of the diode bars. The pattern of emitters in the laser diode bar stack is measured and a unique lens holder is fabricated. Individual gradient-index cylindrical fast-axis collimator lenses are then located in the holder, with one lens per diode bar. In this way, the dominant irregularities in the geometry of the laser diode stack are replicated within the fast-axis array, and consequently, the mismatch between the stack and lens array is significantly reduced. This approach can therefore be used for high fill-factor collimation and can be used on arrays with small (<1.2 mm) fast-axis pitch.
There are, however, significant disadvantages in this approach. For large stacks, fitting these discrete collimators into the array is a time-consuming task, and typically this is done either manually or using a semi-automated system that still requires human input. The collimators must also be fixed in position, usually by epoxy, and thus a curing time is introduced which slows construction. Consequently, the cost of assembly and reliability in terms of ruggedness and environmental stability is undesirable. A further disadvantage is that the lens holder is populated with standard collimators i.e. fixed focal length and pitch, which limits the optimum beam correction. Further, this approach can only correct errors in fast-axis and slow-axis emitter position that vary linearly along each bar, and so it cannot correct for smile or facet bending.